The present invention is concerned with the electrolytic production of chlorine dioxide from chlorite ions. More particularly, the present invention relates to an electrochemical process and an electrolytic cell structure used to manufacture a high purity aqueous chlorine dioxide solution from a dilute aqueous alkali metal chlorite solution.
It is known to produce chlorine dioxide electrolytically by electro-oxidation of chlorite ions according to the following reaction:
ClO2xe2x88x92ClO2+exe2x88x92xe2x80x83xe2x80x83[1]
U.S. Pat. No. 2,163,793 describes an electrochemical chlorine dioxide generating process in which an aqueous solution of alkali metal chlorite and alkali metal chloride is electrolyzed in an electrolytic cell equipped with a porous diaphragm separating the anode and the cathode compartments.
Similar divided electrochemical cells equipped with various types of separators were also employed in numerous other patented processes for electro-oxidation of chlorite ions to chlorine dioxide.
For example, U.S. Pat. No. 2,717,237 discloses a method for producing chlorine dioxide by electrolysis of chlorite in the presence of water-soluble alkali metal sulfate, for example, sodium sulfate.
Japanese Patent Publication 56-158883 published Dec. 7, 1981, (U.S. Pat. No. 4,432,856) describes an electrolytic process for producing chlorine dioxide by oxidation of chlorite in which the electrolyzed solution, at a pH of 2 or less, is fed to a stripping tank where air is introduced to recover the chlorine dioxide.
U.S. Pat. No. 4,542,008 describes an electrolytic process for chlorine dioxide production in which the sodium chlorite concentration of the solution leaving the anode compartment is monitored by means of a spectrophotometric measurement.
Published PCT International Patent Application No. WO 91/09158 and the corresponding U.S. Pat. No. 5,106,465 disclose a method of producing chlorine dioxide from alkali metal chlorite in an ion-exchange compartment of a multi-compartment cell in which hydrogen ions generated in the anode compartment enter the ion-exchange compartment through a cation-exchange membrane, causing chlorite ion disproportionation resulting in the formation of chlorine dioxide.
PCT Published International Patent Application No. WO 94/26670 discloses a method of producing chlorine dioxide from sodium chlorite in which the gaseous product along with water vapor is removed from the electrolyzed solution by means of a microporous, hydrophobic gas membrane.
By removing water at the rate of its input to the anolyte, a continuous, environmentally innocuous operation with no undesired effluent can be effected.
While all the above mentioned patents and patent applications require the recirculation of the electrolyzed solution, PCT Published International Patent Application No. WO 91/09990 and related U.S. patents (U.S. Pat. Nos. 5,041,196; 5,084,149; 5,158,658; 5,298,280 and 5,294,319) teach an electrochemical process for producing chlorine dioxide from a dilute alkali metal chlorite solution in a single pass mode, i.e., with no recirculation of the anolyte, using a porous, high surface area anode. The product solution, in addition to chlorine dioxide, may also contain unconverted chlorite as well as undesired by-products resulting from inefficiencies, such as chlorate ions or chlorine.
An improved single pass operation is disclosed in U.S. Pat. No. 6,203,688 in which the chlorite containing feed solution is pre-acidified prior to its entering the anode compartment of the electrochemical cell.
While the overall performance parameters, i.e., chemical efficiency and product purity, of the above described single pass processes for the electro-oxidation of chlorite ion to chlorine dioxide are generally satisfactory, the complexity of the divided electrochemical cell creates serious operational problems, such as membrane failure and/or anolyte by-pass, as well as negatively affecting the process economics. High sensitivity of the cation-exchange membranes towards certain impurities, primarily hardness, imposes additional requirements on the feed purity, thus further increasing the overall cost of the chlorine dioxide production by the single-pass process.
Recently, after the priority date of this application, U.S. Pat. No. 6,306,281 was issued, describing the concept of chlorine dioxide generation in an undivided electrochemical cell. According to this patent, chlorite conversions of up to 90% can be achieved in a single pass when operating an undivided electrochemical cell with buffered electrolyte having a pH below 10 and preferably in the pH range 8 to 9. Such conversions are considered to be unsatisfactory in commercial operations.
Commercially-acceptable conversion values are typically well above 90% and preferably close to 100%, corresponding to a complete utilization of the reactant. The cost of the reactant sodium chlorite constitutes the main cost of the entire operation.
There is a need, therefore, to develop a electrolytic chlorine dioxide generation process based on a single pass mode with no recirculation of the anolyte wherein the electrochemical cell does not exhibit the above-described deficiencies of the electrolysers known in the art.
Surprisingly, it has been found that electro-oxidation of chlorite ions to chlorine dioxide can be carried out with satisfactory efficiency and with conversions of close to 100% in an undivided electrochemical cell. This finding is completely unexpected, since it was always assumed in the prior art that the presence of a separator in the electrochemical cell is absolutely necessary in order to prevent the occurrence of the decomposition of the product of the anodic reaction, i.e., chlorine dioxide, at the cathode and to ensure the substantially complete utilization of the chlorite reactant.
It is well known that, under cathodic conditions, chlorine dioxide readily undergoes reduction to chlorite ions:
ClO2+exe2x88x92ClO2xe2x88x92xe2x80x83xe2x80x83[2]
which is a reverse reaction to that occurring at the anode (reaction [1]). In addition, it was always assumed that the presence of a separator in the electrochemical cell is beneficial, since it enables the possibility to co-generate sodium hydroxide in the cathodic compartment of a divided cell, thus making the process particularly useful for applications utilizing both ClO2 and NaOH, for example, pulp bleaching. Moreover, it was assumed in the prior art that, in the absence of a separator, the products generated at the anode (ClO2) and the cathode (NaOH) would interact with each other according to the following reaction:
2ClO2+2NaOHNaClO3+NaClO2+H2Oxe2x80x83xe2x80x83[3]
resulting in a significant ClO2 yield loss as well as the formation of a highly undesired by-product, sodium chlorate (NaClO3).
According to the present invention, it was found that the lack of a physical barrier separating the cathodic and anodic compartments of an electrochemical cell does not necessarily preclude the possibility of achieving an effective conversion of chlorite ions to chlorine dioxide and that chlorine dioxide can be produced in a single pass in an undivided electrochemical cell.
Accordingly, in one aspect of the present invention, there is provided a process for the oxidation of chlorite ions to chlorine dioxide in an undivided electrochemical cell operated in single pass, comprising:
(1) passing an aqueous alkali metal chlorite solution through a high surface area anode structure into the interelectrode gap zone between the anode and a cathode;
(2) passing water or an aqueous acidic solution along the cathode structure within the interelectrode zone;
(3) applying sufficient electric current between the anode and the cathode in order to oxidize substantially all the chlorite ions in said aqueous alkali metal chlorite solution to chlorine dioxide;
(4) withdrawing an aqueous solution of chlorine dioxide from the electrochemical cell, optionally into a water stream.